Magnetic resonance tomography as an imaging method is known from the prior art, in which the hydrogen density and binding conditions in an object under examination are determined on the basis of an excitation of the nuclear resonance of the protons in the nuclei of the hydrogen in an external magnetic field B0 by means of an external radio frequency signal and a radio frequency measurement signal emitted thereupon by the object under examination and converted into a pictorial representation of the object under examination. In such cases all water-retaining tissue can be recorded.
The quality of the radio-frequency measurement signal and the images generated therefrom increases in such cases with the strength of the applied static magnetic field B0, since, as the field B0 increases, the energy gap of the states of the proton in the magnetic field increases. This leads, in thermal equilibrium, to an occupancy difference and thus to the signal strength becoming greater. Typical field magnets for magnetic resonance tomography nowadays have magnetic fields of between 0.3 T and 1.5 T, devices with 3 T are also already available.
These are set against regulatory restrictions for magnetic fields in order to restrict the magnetic field strengths to which people are exposed over a long period. This relates in particular to the operating personnel who, by contrast with patients, spend day after day in the vicinity of the field magnets. A typical legal limit amounts to 0.5 mT, but can vary from country to country.
To comply with these limit values it has long been usual to use shields made of ferromagnetic material with a high magnetic permeability, such as soft iron for example. In such cases the limit value can be complied with on the one hand by large material thicknesses of the shield or by a greater distance to the field magnet. Especially with mobile use of magnetic resonance tomographs however, as is described in publication US 2006/0186884 A1, limits are imposed both by the permitted overall weight and also by the external dimensions.
As a weight-reduction and space-saving method for active shielding of a superconducting field magnet, it is known from publication DE 3301630 A1 that the field magnets can be surrounded with superconducting coils around their outer circumference, which generate an opposing magnetic field, and through this reduce the resulting magnetic field in an external space of the magnet apparatus, so that the ferromagnetic shield can be embodied thinner.
The magnetic shield by a ferromagnetic surrounding construction however interacts with the magnetic field B0. The stronger the magnetic field lines run in the external space of the field magnets in the ferromagnetic material, the better is the shielding effect however. At the same time however the coupling of the shielding to the magnetic field increases. This leads to changes in the magnetic shielding also changing the magnetic field B0, especially in the inner chamber of the field magnet, which is required to be highly constant in the spatial and temporal respect in order not to disturb the measurement signal and to achieve an optimum measurement result or an optimum image quality. Changes, which stem from a mechanical load or a change in temperature, can be caused for example by a spatial displacement or deformation of the ferromagnetic shield.